Conditional jumps (except the JCXZ instruction) test the flags that have been set by a previous instruction. The conditions for each mnemonic are given in parentheses after each description above. The terms "less" and "greater" are used for comparisons of signed integers; "above" and "below" are used for unsigned integers.
If the given condition is true, a jump is made to the location provided
as the operand. Instruction coding is most efficient when the target for
the conditional jump is in the current code segment and within -128 to +127
bytes of the next instruction's first byte. The jump can also target -32768
thru +32767 (segment size attribute 16) or -231 thru +231 - 1 (segment size
attribute 32) relative to the next instruction's first byte. When the target
for the conditional jump is in a different segment, use the opposite case
of the jump instruction (i.e., the JE and JNE instructions), and then access
the target with an unconditional far jump to the other segment. For example,
you cannot code- JZ FARLABEL
You must instead code- JNZ BEYOND
JMP FARLABEL
BEYOND:
Because there can be several ways to interpret a particular state of the flags, ASM386 provides more than one mnemonic for most of the conditional jump opcodes. For example, if you compared two characters in AX and want to jump if they are equal, use the JE instructions; or, if you ANDed the AX register with a bit field mask and only want to jump if the result is 0, use the JZ instruction, a synonym for the JE instruction.
The JCXZ instruction differs from other conditional jumps because it tests the contents of the CX or ECX register for 0, not the flags. The JCXZ instruction is useful at the beginning of a conditional loop that terminates with a conditional loop instruction (such as LOOPNE TARGET LABEL. The JCXZ instruction prevents entering the loop with the CX or ECX register equal to zero, which would cause the loop to run 64K or 23² times instead of zero times.